In configurable integrated circuits, various components, such as transistors, resistors, junction capacitors, diodes, and zener diodes, each component having a variety of fixed values, are distributed on a single chip and not connected by any metallization layer. In a typical configurable integrated circuit where the user defines the metallization layer interconnecting the various components, the user selects various circuits and sub-circuit elements from a library of circuit elements to be used as building blocks in the design of a single chip system. The user may also design custom circuits using the various components available on the configurable chip. A software model of the various circuits and elements connected together is then created, and the system is simulated to see how the circuits and elements operate as a system.
Once the entire system is settled upon by the user, and the various interconnections between the individual components are established, a diskette or other memory device containing the user's desired interconnections is then used by the configurable integrated circuit manufacturer to automatically create a metallization mask. The configurable integrated circuit chip then undergoes a single photolithographic mask process and metallization process to form the interconnections between the various components on the chip.
A configurable integrated circuit manufacturer designs a particular class of chip to have a particular die size and incorporates as many components as feasible within that die size to give the user a large number of design options. Typically, each component is made separate so that if an individual component is not used, that portion of the die area taken up by that unused component is essentially wasted.
One such manufacturer of configurable integrated circuits is Custom Array Corporation in Sunnyvale, Calif. A configurable integrated circuit supplied by Custom Array Corporation enables the user to form zener diodes from the base and emitter regions of transistors. Thus, in this design where a zener diode and a transistor are formed from the same diffused regions, the characteristics of the zener diode are totally dependent upon, and subservient to, the design of the transistor. Consequently, the breakdown voltage of the zener diode is most likely not a value which would be optimum for a user. One advantage in this type of design, however, is that the zener diode requires no additional real estate and requires no additional wafer processing.
Typically many high breakdown voltage zener diodes are required in a power related integrated circuit for voltage regulation. If the zener diodes formed by the base and emitter regions of an existing transistor exhibit low breakdown voltages, a number of zener diodes must be connected in series if it is desired to achieve a resulting higher breakdown voltage. In the case of the Custom Array Corporation zener diode, using serial combinations of low breakdown voltage zener diodes greatly reduces the number of available transistors for other purposes.
FIG. 1 shows a prior art device which may be included in a configurable integrated circuit. Such a device is shown in U.S. Pat. No. 4,589,004 to Yasuda et al., incorporated herein by reference. In this device, a vertical DMOS device 4 uses a heavily doped sinker region 6 to contact a heavily doped buried layer 8 or a heavily doped substrate. The sinker and buried layer also act to lower the beta of any parasitic transistors. In FIG. 1, this sinker region and buried layer act as a drain region of vertical DMOS transistor 4, wherein highly doped sinker region 6 is exposed for eventual contact during metallization.
In the prior art, where a configurable integrated circuit incorporates a vertical transistor using a sinker region, such as shown in FIG. 1, any zener diode also included in the configurable integrated circuit is either a separate and independent device, using additional die area, or is formed using existing regions of a transistor, where the breakdown voltage of the zener diode is dictated by the design of the transistor.
A conventional zener diode (the term zener being used to also include avalanche diodes) is shown in FIG. 2, where a cathode region 9 is formed in a semiconductor substrate, and an anode region 10 is formed within the cathode. A zener diode may also be formed where the cathode region and the anode region are made side by side. When the zener diode is sufficiently reversed biased to cause breakdown, electron-hole pairs are created to thereby support a current, thus causing a voltage drop across the diode equal to its breakdown voltage.
Additional zener diode structures are shown in U.S. Pat. Nos. 4,758,537; 4,742,021; 4,735,210; 4,419,681; 4,293,868; and 3,735,210, all incorporated herein by reference.
What would be desirable in the industry is a zener diode structure for use in configurable integrated circuits which does not require additional die area in any way and which may be independently designed to have a desired breakdown voltage.